It is known to reinforce the structure of an airplane to enable it to receive on its floor, not passengers, but rather loads such as pallets. The structure then needs to support the inertial forces that are generated by the pallets when they are subjected to the accelerations of the airplane and that are transmitted to the structure of the airplane.
This problem affects the central wing box most particularly. The wing box is already strongly stressed by forces coming from the wings. Thus, accompanying FIGS. 1 and 2 show a box section in a vertical plane perpendicular to the longitudinal direction of the fuselage. The box 2 has a bottom panel 4 and a top panel 6 extending above the bottom panel. These two panels are generally plane and horizontal, being spaced apart from each other. They are connected to each other by frames 8 having the wings 10 of the airplane attached thereto. The remainder of the fuselage 12 extends above the box 2. The wings subject the top panel 6 to compression forces FE and the bottom panel 4 to traction forces F1. A shear force T appears at the junction between the wings and the frames, which force is balanced by the frames. Each of the wings also generates a bending moment Mf on the box.
Accompanying FIG. 3 shows the top panel 6 in greater detail in a similar view. The atmospheric pressure P inside the cabin of the airplane also acts on the top panel. Thus, this panel is subjected simultaneously to compression forces F and to pressure forces P that are uniformly distributed. This set of stresses applied to the panel and its stiffeners makes the panel particularly sensitive to instability. Under such conditions, it is desirable not to add any shear force (a force acting along the vertical axis of the airplane) and not to add any bending moment between the two bearing points at the ends of the panel.
Finally, the relative deformation of the central wing box can be considerable. Thus, accompanying FIG. 4, once more in vertical section, shows one lateral half of the box 2, with continuous lines showing its shape when it is not subjected to stresses, and with chain-dotted lines showing its shape when it is deformed under the effect of said stresses. The top panel 6 tends to sag downwards, as does the bottom panel 4. Nevertheless, the top panel has its ends moved towards each other while the bottom panel has its ends pulled apart from each other. The frames are subjected to turning through an angle □ about an axis parallel to the longitudinal direction of the fuselage.
Consequently, any structural reinforcements that might be made must not add excessively to the stiffness of the box so as to allow it to deform in this way, since otherwise the movements imposed on the box would load the new structural elements and their environment excessively.
Consideration may also be given to the positions of pallets and their anchor points. Accompanying FIG. 5 is a section view in a vertical plane perpendicular to the longitudinal direction of the airplane and showing the central wing box together with two loaded pallets 16 received on the floor 14 above the box. In general, the anchor points for the pallets are situated at their lateral sides, at left, central, and right anchor points 18, 20, and 22. At each of these anchor points, forces in the vertical and lateral directions are introduced into the floor. The forces introduced via the lateral points 18 and 22 can be balanced without difficulty. In contrast, it is difficult to balance the forces introduced via the central point 20, while still complying with the constraints associated with the environment of the central wing box.